Sharif Digital Repository

Luminescence down shifting (LDS) of the incident spectrum is a practical strategy that helps to increase the light harvesting efficiency by manipulating the incident spectrum instead of interfering with the active material inside the cell. In this research, for a dye-sensitized solar cell with a near-infrared sensitizer, the photovoltaic performance was enhanced remarkably with a reflective luminescent down-shifting (R-LDS) layer. Different inorganic phosphors with different luminescent quantum efficiencies have been used as the down shifting materials to increase the light-harvesting efficiency of DSSCs sensitized with TT1 and SQ1 dyes. Four different structures were examined to find the... 

Epsilon-near-zero media have attracted considerable attention in the last decade due to their exceptional behavior in wave propagation. The purpose of this research project is to analyze time-periodic ENZ structures. To this end, layered structures consisting of plasmonic metal and dielectric layers are used to achieve the epsilon-near-zero effect in a specific frequency range, and by time-variations of the dielectric layers, an effective time-varying (TV) ENZ is achieved. A method based on transfer matrix method is developed to analyze such structures while they are undergoing time-variations, and primarily under normal incident waves. Various structures based on such TV layered structures... 

We study the phenomenon of parametric amplification and instability in the context of time-varying electromagnetic structures, particularly dielectric slabs with periodically modulated permittivity. These structures show particular promise inasmuch as they are capable of very large amplifications when illuminated by an electromagnetic wave of half the modulation frequency. Successive studies have corroborated this finding but none have yet been able to ascertain the nature of amplification in such devices. On top of that, the problem of instability in time-periodic slabs has either been disregarded altogether or it has been subjected to loose analysis, leading to mere speculations which are... 

This thesis presents a resonant time-periodic structure based on the Fabry-Perot resonator for frequency comb generation. Time-periodic structures cannot generate a frequency comb by themselves because side-harmonic amplitudes are really weak. However, due to resonant structure, it is expected that the side-harmonics, which are generated because of the time-periodic part of the structure, are increased if they are close to the main modes of the structure. But the results show that, generally, the amplitude of side harmonics, which are close to the main mode, is increased and far side-harmonics don't have remarkable amplitudes. However, we will show that if the modulation frequency of the... 

Over the past few years, time-periodic and space-time-periodic media have garnered a considerable amount of attention, thanks to their exotic and unique interactions with electromagnetic phenomena. These media do not generally comport with Lorentz's reciprocity theorem, thus can be utilized in order to implement non-reciprocal amplification and frequency transformation. One class of such media, that have been the subject of less discussion and interest, are space-time periodic diffraction gratings.In this thesis, we aim to develop a series of semi-analytical methods in order to analyze such structures. Our focus will be on a subset of methods which make use of the modal expansion of fields... 

In this thesis, we study optomechanics of subwavelength nanoparticles based on the Green's function formulation. First, we investigate the optical force exerted upon Rayleigh particles in the free space using the dipole approximation method. Then, we present a new method based on the Taylor expansion of the polarization field to calculate the optical forces beyond the Rayleigh regime. Subsequently, we study the optical force exerted upon Rayleigh particles in non-free spaces, and model the backaction effect using the scattering Green's function. We show that the backaction effect can modify the polarizability of the particle and thereby can affect the gradient force, radiation pressure, and... 

Electronic processors have encountered Moore’s limitations and they can not meet expectations following computer science advances in new algorithms such as neural networks. These limitations have brought research to focus on light as a high-capacity carrier of information. Light’s degrees of freedom can be exploited whenever the structure has devices able to carry, keep, manipulate and detect each DoF's information but this exploitation may itself lead to either a large footprint or more power consumption. Moreover, there are some scientific and technological difficulties in matching different computational algorithms to these DoF’s requirements. As a result, one must consider all advantages... 

Researchers have long considered the phenomenon of parametric amplification. This phenomenon is introduced first in nonlinear structures and then in limited systems of time-varying linear structures. Time-modulated systems have many capabilities, such as amplification, non-reciprocity, and other phenomena in wave propagation. So far, time-varying circuits have been studied in terms of stability and analytical solution. The purpose of this project is to investigate parametric amplification in time-varying linear systems. Studying a structure under a circuit model is essential to other analyses practically implemented in various frequency ranges from microwave to light. As a result, this... 

Time-varying media have been in center of attention since recent years for their plethora of applications. Achieving non-reciprocity, one-way propagation, frequency modulation, and signal amplification are some applications of such media which are accessible from microwave to optical frequencies. The aim of this thesis is to first find a method for the analysis of wave propagation in homogeneous time-varying media. To this end, a new formulation is presented based on differential transfer matrices enabling us to find amplitudes, average power, and average energy of a wave in a homogeneous time-varying medium for arbitrary temporal variation of permittivity. In addition,this formulation... 

The electromagnetic (EM) force is the pivot on which this thesis revolves. We inspect different formulations of the distribution of the EM force (and momentum) inside matter, and show that the Einstein-Laub force density is incompatible with special relativity. We study the exerted EM force on small particles in depth, and explain when the conventional dipole approximation fails to yield accurate results. We propose an all-dielectric structure which is able to trap very small dielectric particles (of diameters as small as 10 nm). One of the achievements of this thesis is an explanation for an adverse phenomenon observed in many experiments on the optomechanical systems employing... 

Artificial neural networks are systems based on the brain’s functionality which in many cases are able to process highly complex computational tasks like speech recognition, image recognition ,and time series prediction. Due to the complexity of training algorithms in recurrent neural networks, reservoir computers have significant importance in machine learning.Due to low power consumption and crosstalk, high bandwidth and high-speed computing in optics, reservoir computing has proceeded to optical implementations. A reservoir computer consists of three layers: the input layer, reservoir, and output layer. A recurrent neural network is usually used as the reservoir in reservoir computers.... 

In this thesis, study of some kinds of dual band transmission lines and structures are investigated. Attention to 3 important kinds, CRLH, SCRLH and the equal phase line are introduced and the formulations of each one are derived and considered. Also the method of designing these lines are attached. These lines form a complete category that makes the ability of designing all kind of dual band line with arbitrary frequencies and result phases with arbitrary non-integer ratio less or more than 1. Also an important attention is paid for the designing these lines in the microstrip configuration. Various kinds of Dual band structures are proposed at the last part of the introducing each lines.... 

Structures with time-varying electromagnetic properties can potentially yield non-reciprocity, amplification, harmonic generation, and other interesting wave propagation phenomena. To date, some studies have dealt with time-varying media and circuits, including time-varying optical ring resonators having degenerate travelling wave resonances. In time-varying ring resonators, the clockwise and counter clockwise resonances lose their degeneracy, and non-reciprocity is observed. One of the main challenges of such research is the need to induce time-variations in the entirety of the resonator or structure, which makes their implementation very limited or impossible in practice. The purpose of... 

Reservoir Computing is a novel computing paradigm that uses a nonlinear recurrent dynamical system to carry out information processing. Recent electronic and optoelectronic Reservoir Computers based on an architecture with a single nonlinear node and a delay loop have shown performance on standardized tasks comparable to state-of-the-art digital implementations. Here we report an all-optical implementation of a Reservoir Computer, made of off-the-shelf components for optical telecommunications. It uses a semiconductor optical amplifier as nonlinearity, and a Fabry-Perot Resonator as a key element to establish the virtual nodes, connecting them and consequently, build the virtual neural... 

In this thesis, while a comprehensive study of different methods for the characterization of the optical fibers is done, a unique and effective method is being introduced for the characterization of the dispersion coefficient of Highly Nonlinear Fibers (HNLFs). The proposed method is based on the Brillouin Optical Time Domain Analysis (BOTDA) of a wave generated by the Four Wave Mixing (FWM) interaction. The current method, which includes an experimental scheme and an algorithm for solving the inverse problem, offers high sensitivity and experimental accuracy at the longitudinal resolution of 1 meter. The noise level has been considerably reduced by understanding different sources of the... 

Subwavelength localization of electromagnetic energy with intense local fields, also known as electromagnetic hotspots, has received significant attention over the past few decades. In most cases the hotspot is achieved through the resonant concentration of electromagnetic fields. One way recently considered to get hotspot is through reflection of electromagnetic waves in nonreciprocal one-way structures, in plasmonics and via magneto optic effect.However these hotspots are less confined compared to hotspots caused by resonances. So combining nonreciprocal structures with resonance can provide better hotspots.Our aim in this thesis is to study nonreciprocal resonant structures using coupled... 

Despite of significant developments of optical trapping in past decays, trapping of small nanometer-sized particle faces some difficulties. Optical force reduces rapidly by decreasing the size of the particle because of reducing the particle’s polarizability. So, it is necessary to increase the intensity of incident beam to attain a stable trap. However, it can lead to damage of particle before getting trapped. Self induced back-action(SIBA) is one of the proposed solutions to overcome this problem. In this phenomena particle plays an active role in trapping by changing the surrounding electromagnetics field. In this thesis, we show that SIBA is a phenomenon that appears when we extend the... 

Due to the growing need for plasmonic wavs in microwave and terahertz spectra, a periodic arrangement of one-dimensional cut-through slits is investigated and an equivalent model based on the effective medium theory is derived. In contrast to the all previous attempts that were successful in mimicking only the zeroth-order diffracted waves, the proposed effective medium is capable of mimicking all diffraction orders. The parameters of the equivalent model are established by comparing the scattered waves of the semi-homogeneous medium and those of the main structure obtained by invoking the rigorous mode matching approach based on the single mode approximation inside the slits. This medium is... 

Thus far numerous components and devices have been designed and realized based on plasmonics. Hereon, we focus on PlasMOStor which is probably the first and most important CMOS compatible plasmonic modulator. It resembles to its electronic counterpart MOS transistor in geometry, unless, it has a plasmonic waveguide instead of the regular channel to modulate optical signals . By applying voltage to the gate and inset of charge carrier accumulation, optical properties of the channel changes and plasMOStor turns off. In this thesis, we carry out a through investigation of plasMOStor’s operation. We show that the proposed theoretical description of plasMOStor is fallacious. Subsequently,... 

In this thesis we investigate the use of periodic array of metallic structures which mimics the propagation of the surface plasmon-polaritons waves. The TEM modes supported by these structured surfaces are referred to as the spoof surface plasmon-polaritons. They have many properties in common with the electron plasma and can be described by a plasma-like effective permittivity, when the structure is on a scale much smaller than the wavelength. In this thesis we extend this concept to include different kinds of structured surfaces like cut through metallic slits array and square array of rectangular metallic pillars which support TEM modes. We show that although these structured surfaces...